extension: wire extension registries into sessions (#21737)

## Why

[#21736](https://github.com/openai/codex/pull/21736) introduces the
typed extension API, but the runtime does not yet carry a registry
through thread/session startup or give contributors host-owned stores to
read from. This PR wires that host-side path so later feature migrations
can move product-specific behavior behind typed contributions without
adding another bespoke seam directly to `codex-core`.

## What changed

- Thread `ExtensionRegistry<Config>` through `ThreadManager`,
`CodexSpawnArgs`, `Session`, and sub-agent spawn paths.
- Wire `ThreadStartContributor` and `ContextContributor`
- Expose the small supporting surface needed by non-core callers that
construct threads directly, including `empty_extension_registry()`
through `codex-core-api`.

This PR lands the host plumbing only: the app-server registry is still
empty, and concrete feature migrations are intended to follow
separately.

codex-rs/Cargo.lock

@@ -1895,6 +1895,7 @@ dependencies = [
  "codex-core",
  "codex-core-plugins",
  "codex-exec-server",
+ "codex-extension-api",
  "codex-external-agent-migration",
  "codex-external-agent-sessions",
  "codex-features",
@@ -2497,6 +2498,7 @@ dependencies = [
  "codex-core-skills",
  "codex-exec-server",
  "codex-execpolicy",
+ "codex-extension-api",
  "codex-features",
  "codex-feedback",
  "codex-git-utils",
@@ -2600,6 +2602,7 @@ dependencies = [
  "codex-config",
  "codex-core",
  "codex-exec-server",
+ "codex-extension-api",
  "codex-features",
  "codex-login",
  "codex-model-provider-info",
@@ -3100,6 +3103,7 @@ dependencies = [
  "codex-config",
  "codex-core",
  "codex-exec-server",
+ "codex-extension-api",
  "codex-login",
  "codex-protocol",
  "codex-shell-command",
@@ -4307,6 +4311,7 @@ dependencies = [
  "codex-config",
  "codex-core",
  "codex-exec-server",
+ "codex-extension-api",
  "codex-features",
  "codex-hooks",
  "codex-login",